Optical Identification of Gamma-Ray Blazars
An optical image of a field of galaxies, all potentially responsible for the gamma-rays detected in this region by Fermi since all lie within Fermi's large field-of-view (white ellipse). Blazars are a dominant source of gamma-rays, but optical spectra are needed to identify and categorize a galaxy as being a blazar and there are usually too many galaxies in Fermi fields to followup all its gamma-ray detections. Astronomers have found that the infrared colors of blazars can be used to identify likely candidates in infrared images of these fields. Red circle marks the identified blazar gamma-ray emitter; green crosses mark bright radio galaxies.
Blazars are galaxies whose central, supermassive black holes are accreting material from surrounding regions and emitting powerful beams of high velocity charged particles that coincidentally are pointed in our direction. The charged particles include electron that can produce gamma ray photons, with each photon packing over a hundred million times the energy of the highest energy X-ray photons seen by the Chandra X-ray Observatory. The electron jets in blazars also exhibit rapid and strong variability.
One of the main challenges in modern gamma-ray astronomy is the identification of the exact sources of strong gamma-ray emission seen by the Fermi Large Area Telescope. Blazars fall primarily into two classes distinguished on the basis of their optical spectra: those lacking prominent emission lines, and those with strong, broad lines. Pinpointing Fermi sources would enable followup observations to determine if the galaxy is a blazar or something else, and if it is a blazar, what type it is. Optical spectra could also measure the source’s redshift and thereby establish the absolute luminosity of the galaxy. The problem is that the location of a Fermi source in the sky is very uncertain, typically by about four arcminutes (about one-tenth the size of the full moon), and dozens of possible galaxies or more can lie in a region of this size. Of the over five thousand gamma-ray sources discovered by Fermi so far, nearly 30% have no obvious optical counterpart.
CfA astronomers Raffaele D'Abrusco and Howard Smith are members of a blazar team that realized that the infrared color of blazars as seen with the WISE (Wide-field Infrared Survey Explorer) filters are distinctive, and that WISE images of the Fermi field can therefore often spot candidate blazars for subsequent optical confirmation. The team recently released two catalogs of WISE-identified blazar candidates, and decided to search the Sloan Digital Sky Survey archive of spectra for matches. The astronomers examined 1830 Sloan galaxy spectra for blazar signatures, and found that about 30% of them are indeed blazars, with many other candidates being quasars. The team further analyzed the results to categorize the blazar types and evaluate the impact of selection effects. In the coming era of the James Webb Space telescope and very large ground-based telescope it should be possible to obtain diagnostic spectra of even fainter candidate galaxies.
"Optical Characterization of WISE Selected Blazar Candidates," Raniere de Menezes, Harold A. Peña-Herazo, Ezequiel J. Marchesini, Raffaele D'Abrusco, Nicola Masetti, Rodrigo Nemmen, Francesco Massaro, Federica Ricci1, Marco Landoni, Alessandro Paggi, and Howard A. Smith, A&A 2019 (in press).